Originally Posted by dshealey
Not much one can do to keep brake fluid out of the clutch other than the drain hole. Even if you could seal the nose of the cylinder to the bore, the brake fluid comes from inside the cylinder, around the piston when the seal fails, so it would just get funneled nicely by your paste more rapidly into the clutch. At least there is a little "reservoir" for leaking fluid around the nose of the cylinder, putting any other substance there would reduce the leak retaining volume, so would decrease the time before clutch failure.
If the rear transmission input shaft seal, right in front of the slave leaks, transmission fluid is eventually pushed forward into the clutch also. This is normally a slow leak though, so can exist for quite some time before enough collects to be pumped forward by the action of the clutch slave cylinder piston. The failing of the slave cylinder however will push brake fluid out at a higher rate, so the drain hole has to be large enough to allow this out instead of being forced into the clutch.
I am of the mind the most likely cause of failure of the slave cylinder will be because of an intrusion of trans. fluid causing damage to the slave cylinder seal 'rubber' and possibly the slave cylinder piston 'rubbers' as well. This in turn will cause brake fluid to be injected under pressure, from the operation of the clutch lever, into the area of the clutch rod and eventually into the clutch.
The reason I am thinking about coating the nose of the slave cylinder is to help keep any trans. fluid leak from directly contacting the slave cylinder seal.
If this can be done, it will help to prevent the failure of the slave cylinder seal from the outside. Thereby eliminating that as a cause for failure of the slave cylinder.
I was concerned that there might not be enough room around the end of the slave cylinder to permit the trans. fluid to drain out the drain hole if the nose of the cylinder is coated. By my measurements, there is about .094" between the end of the cylinder and the end of the machine bore in the housing. There is an additional larger space from the end of the machine bore to the face of the trans. seal that would provide more room for the leaking trans. fluid to accumulate as it migrates to the drain hole.
I agree, most trans. leaks will tend to be slow and the drain hole should be adequate, and provide an early enough warning. The slave seal 'protection' barrier should give more time particularly if there is a substantial trans. fluid leak.
One other thing. I am not sure if the moly paste is the appropriate barrier. A dielectric paste may be suited to that application. I have some brake system 'rubbers. I am going to experiment on and see if either is ok.